Gas-Phase Nitrous Acid (HONO) Is Controlled by Surface Interactions of Adsorbed Nitrite (NO2–) on Common Indoor Material Surfaces

Nitrous acid (HONO) is a household pollutant exhibiting adverse health effects and a major source of indoor OH radicals under a variety of lighting conditions. The present study focuses on gas-phase HONO and condensed-phase nitrite and nitrate formation on indoor surface thin films following heterogeneous hydrolysis of NO2, in the presence and absence of light, and nitrate (NO3–) photochemistry. These thin films are composed of common building materials including zeolite, kaolinite, painted walls, and cement. Gas-phase HONO is measured using an incoherent broadband cavity-enhanced ultraviolet absorption spectrometer (IBBCEAS), whereby condensed-phase products, adsorbed nitrite and nitrate, are quantified using ion chromatography. All of the surface materials used in this study can store nitrogen oxides as nitrate, but only thin films of zeolite and cement can act as condensed-phase nitrite reservoirs. For both the photo-enhanced heterogeneous hydrolysis of NO2 and nitrate photochemistry, the amount of HONO produced depends on the material surface. For zeolite and cement, little HONO is produced, whereas HONO is the major product from kaolinite and painted wall surfaces. An important result of this study is that surface interactions of adsorbed nitrite are key to HONO formation, and the stronger the interaction of nitrite with the surface, the less gas-phase HONO produced.

. Change in gas-phase HONO or NO2 concentration from photo-enhanced NO2 hydrolysis reaction in presence of A) kaolinite, B) cement, and C) CH3COOH exposed cement samples. Thin films were exposed to a fixed quantity of NO2 (~110 ppb) at 45±5 % RH in the presence of solar light. For kaolinite, NO2 level depleted by ~15 ppb and approximately makes 5 ppb of gas-phase HONO under steady -state condition. For the cement sample, gas-phase HONO concentrations were below the background HONO mixing ratio.
S-3 subsequent HONO formation under illumination are presented in Figure S1. A large initial uptake of NO2 was observed that slowly reaches steady-state condition. Heterogeneous reaction of NO2 on particle surface results in the formation of NO, which was not probed in this study. A total of 3.9 ± 0.5 × 10 16 molecules and 9.9 ± 0.5 × 10 16 molecules of NO2 were adsorbed by the kaolinite and cement surfaces, respectively over a period of 16 h. Steady state concentration of HONO was ~5 ppb above the background signal for kaolinite surface which is equivalent to 1.3 × 10 16 molecules. HONO concentrations from the cement surface film was below the signal to noise ratio.
Condensed-phase nitrate and nitrite concentrations are summarized in Table S1. Taking all of this into account, we estimate that approximately 15-20% of products are other nitrogen-containing products, most likely other gas-phase nitrogen oxides such as NO or N2O. these surfaces were exposed to ~200 ppb of HONO under the dark condition at RH = 45±5 % to determine their efficiency as a HONO reservoir. Figure S2 shows the condensed-phase nitrite coverage following HONO exposure for 16 h. Like NO2 exposed samples, nitrite concentration S-4 was significant on zeolite and cement proxy surfaces. This result further supports the conclusions that only some surfaces act as nitrite reservoirs.

S.3. The effect of surface nitrate loss over time
HONO production should be a function of the concentration of surface adsorbed nitrate or nitrite unless it is present in large excess. The effect of surface nitrite or nitrate loss on gas-phase HONO concentration was examined by photolyzing NO2 exposed surface for a long time at a fixed reaction condition; solar radiation at RH = 45±5 %. As shown in Figure S3, HONO concentration dropped over time for NO2 exposed painted surface. Gas-phase HONO concentration ([ ] !"#$%&"' ) was fitted to an exponential decay function eq. S1.
[ ] !"#$%&"' = * ()* . 1 Figure S2. Surface coverage of nitrite ions on four different indoor model surfaces zeolite, kaolinite, CaO + CaCO3 as cement proxy, and painted wall following exposure to ~200 ppb of HONO for 16 h in the dark and at RH = 45±5 %. Data points are the average of multiple measurements, and error bars represent uncertainties of ±1s.

S-5
Here parameter A is a proportionality constant, t = time, and k = decay constant. Fitted decay constants for different surfaces are determined to be 0.15+0.01 s -1 for these surfaces. An average of 15-16% correction in concentration was required due to surface adsorbed nitrate loss over time.
The effect of surface nitrate and nitrite loss was countered using eq. S2.

S.4. HONO generation in the dark from surface adsorbed nitrate under humid conditions
RH-dependent HONO production from surface adsorbed nitrate and nitrite was also performed. Figure S4 presents the RH-dependent HONO and NO2 concentrations produced from NO2 exposed surfaces under dark conditions. These concentrations are the average of multiple measurements.
Gas-phase product concentrations are significant differences on different surfaces. NO2 is the dominant product from zeolite and cement proxy samples where kaolinite and painted walls S-6 primarily produce HONO. The maximum amount of gas-phase HONO is being generated on the painted surface (10-60 ppb) followed by the kaolinite surface (5-45 ppb). Gas-phase HONO is the major product for these two samples where the HONO mixing ratio increases with the increase of the RH except for the highest RH value. This trend suggests that the adsorbed water plays an important role in the renoxification reaction to produce HONO from NO2 exposed painted walls and kaolinite surfaces. RH-dependent HONO concentration from the cement proxy sample follows the same trend but the overall HONO concentration is significantly lower (5-10 ppb) than the other two surfaces. Similar to the cement sample, a smaller amount (5-15 ppb) of HONO was produced from the zeolite surface which, follows a different RH dependence; an enhancement up to RH = 30% and drops after that. NO2 is the dominant product for zeolite and cement proxy where its concentration remained the same at all RH. This implies NO2 degassing from these surfaces takes place at a fixed rate which is independent of RH. However, a gradual decrease in NO2 concentration was observed for the painted wall surface. This could imply that surface adsorbed water enhances NO2 hydrolysis to increase gas-phase HONO concentrations. RH-dependent NO2 formation on kaolinite follows a complex trend; initial increase up to RH=30% followed by a dip and a small enhancement >60% RH. A combination of two processes might be occurring simultaneously a) replacement of adsorbed NO2 or N2O4 by water molecules and b) increased water adsorption opens up more reactive sites as previously discussed by Liu et al.

S-8
Alkaline surface materials such as grout and concrete are found to be a good reservoir of nitrite and vinegar solution could alter the surface pH to facilitate the protonation step of reaction RS1 and RS2 or the protonation of Ca(NO2)2. Hence, we propose the following reaction mechanism for HONO formation in the dark: 4 Figure S5. Change in surface coverage of nitrite (black) and nitrate (red) ions for NO2 exposed surfaces after being introduced to RH for 6 h. A negative value represents the depletion, and a positive value signifies growth. For all four surfaces, nitrate was depleted where nitrate ion concentration was enhanced.  and NO2. An enhancement of nitrite concentration was observed for zeolite and cement samples.
HONO generated through reactions RS1 and RS2 could be stabilized as nitrite on the surfaces.

S.5. Photo-enhanced gas-phase HONO production
Changes in the HONO concentration as a function of RH for NO2 exposed samples in the presence of solar radiation are shown in Figure S6. Total HONO and NO2 concentrations are the sum of products from two different mechanism: some produced in the dark as explained in the previous sections (sec. S.4) and some from nitrate photochemistry. Figure 4 in the main text presents the photo-enhanced fraction (DHONO and DNO2) of the gas-phase products. Photo-enhanced HONO concentration increases with the increase of RH as surface adsorbed water facilitates NO2 hydrolysis or protonation of nitrite. It is to point out that multilayer water uptake or liquification on the surface can hinder the water-to-air exchange rate of HONO as previously seen on gypsum surfaces. 1 Additionally, a greater wall loss of highly water-soluble HONO can reduce the gasphase photo products. For zeolite, cement proxy, and painted wall, enhancement is higher than suppression. Kaolinite surface follows a complex trend; initial increase up to RH=45% followed by a dip and a small enhancement >70% RH. This could be the result of two compensating factors that are operating in kaolinite as discussed in the previous section. A combination of two processes might be occurring simultaneously as previously discussed by Liu et al. a) reduction of gas-surface exchange due to surface adsorbed water molecules and b) increased water adsorption opens up more reactive sites. 2 For the cement material, HONO percentage increases with RH except at the lowest RH condition. This can be argued that the surface −OH determines the reaction rate of NO2 hydrolysis at dry conditions. As RH increases, water starts to condense on the surface and makes NO2 hydrolysis more feasible in the liquid phase. Additionally, NO2 hydrolysis at the photoactive S-10 site makes HNO3 which can change the pH of the surface locally and can enhance the protonation of nitrite. Figure S6. Gas-phase HONO (red) and NO2 (cyan) concentrations as a function of RH for solarirradiated NO2-exposed thin films (A) zeolite, (B) kaolinite, (C) CaO + CaCO3 as cement proxy, and (D) painted wall in the presence of solar simulator. Data points are the average of triplicate measurements, and error bars represents a sigma uncertainty.
Measurements of the surface coverage of nitrate and nitrite were also performed on the same NO2 exposed surfaces at the end of the gas-phase photolysis experiments. Figure S7 presents the change in surface coverage by subtracting the surface coverage measured using ion chromatography before and after photolysis where each sample was exposed to light and humidity for 6 h. A negative value represents the loss of surface coverage. Nitrate loss is apparent for zeolite, kaolinite, and painted surface. For zeolite and painted surfaces, nitrate concentration reduces upon irradiation along with a small growth of nitrite coverage. Nitrate photolysis under humid conditions makes nitrite which can be stabilized by zeolite or porous painted surface. Therefore, the loss of nitrate resulted in the growth of nitrite concentration. Alternatively, nitrate can directly convert to nitrite; photoisomerization to peroxynitrite followed by rapid dissociation. . Change in surface coverage of nitrite (red) and nitrate (black) ions for NO2 exposed surfaces after broadband irradiation for 6 h under humid conditions. A negative value represents the depletion of nitrite or nitrate. A positive value signifies growth.

S-12
A drastically different result was found for the cement proxy surface; loss of nitrite coverage and rise of nitrate coverage. This implies that some of the nitrites were converted to nitrate upon photolysis. Here we propose the following pathway which involves the photolysis of nitrite. Nitrite has three distinct UV absorption bands: a π → π* transition around 220 nm and two bands peaking near 318 nm and 354 nm, corresponding to n → π* transition. 6

S.7. Implications for indoor air environments
To estimate the concentration of HONO resulting from irradiated kaolinite clay surfaces, following equation can be used. where /080 ( ) is the gas-phase HONO concentration (mg m -3 ) in a room, /080 is the emission rate of HONO in mg m −2 h −1 , is the surface area of the room (m 2 ), is the room volume (m 3 ), A9B is the air exchange rate (h −1 ), and ( ) is the photolysis rate of indoor HONO. It is worth mentioning that outdoor HONO levels are usually lower than indoor levels and it assumed to be zero in the above expression. Figure S8. The estimated (Eq. S3) HONO concentration from NO3 − photochemistry on kaolinite surface in an indoor air environment when one-fifth of the indoor volume is directly illuminated by sun light and at RH = 45 ± 5 %.. HONO concentration is calculated considering 100% (black), 50% (red), and 30% (blue) of surface materials available for reaction.
Consider an unfurnished room with a total volume of = 50 m 3 , total surface area of 85 m 2 (S/V = 1.7 m -1 ), and total wall and ceiling area made of kaolinite = 55 m 2 . Assuming only one fifth of the room volume is irradiated by direct solar light C = 10 m 3 , an average air exchange rate A9B = 0.56 h -1 , and HONO photolysis rate ( ) = 0.26 h -1 , HONO concentration is predicted in Figure 5 in the main text. In this calculation and in the condensed-phase data analysis, it is assumed that the entire surface materials were involved in the reaction. Indoor HONO concentration is S-14 calculated considering 100%, 50%, and 30% of surface materials being reacted for kaolinite sample.